1. Field of the Invention
This invention relates to sludge treatment and, in one aspect, to treating waste activated sludge.
2. Description of Related Art
Industrial waste containing organic pollutants can be treated with pollutant-removing microorganisms which remove organic matter from liquid waste by synthesis into new protoplasm. The microorganisms use degradable organic matter for food. Oxidation of an organic substrate to carbon dioxide produces energy for the microorganism's metabolic functions.
In biological treatment of wastewater, microorganisms consume dissolved and colloidal organic matter, converting that organic matter to new microbial mass, gaseous products (e.g., carbon dioxide), and some new organic compounds. In many systems, there is a net production of microbial mass. In one form of biological treatment-activated sludge systems--the microorganisms are contained in an aqueous slurry and are kept aerobic. The excess microbial mass produced from activated sludge systems is commonly called waste activated sludge (WAS). WAS is usually very difficult to mechanically dewater and often difficult to dispose of. One problem is how to treat and dispose of excess biological solids such as waste activated sludge.
There are a variety of prior art methods for sludge treatment. Heat treatment (originally called the Porteous process) of sludges involves heating sludge to temperatures (in degrees Centigrade) above 100 degrees, generally 180-240 degrees, in a pressurized vessel for a selected time, generally 15-40 minutes, in order to improve the dewaterability of the sludge solids. Heat treatment adds no air or oxygen for oxidation. Some solids are solubilized in heat treatment. That is, hydrolysis breaks long-chain organic molecules into shorter-chain organics; contents of disrupted microbial cells are released; and the net effect is that suspended matter is converted to dissolved and colloidal matter. This produces a high-strength stream which must be returned to the biotreatment system.
A known chemolysis process is a hot acid hydrolysis process for sludge treatment. It is similar to heat treatment under acidic conditions, but temperature is reduced below usual heat treatment levels; e.g., 135 degrees Centigrade to 150 degrees Centigrade. In chemolysis, thickened sludge is acidified to a pH between 0 and 1, heated by steam injection to about 135 degrees Centigrade, and allowed to react for 2 hours in a lined pipe reactor. The stream is then neutralized with caustic, quenched with cooling water to 40 to 45 degrees Centigrade, and settled in a gravity thickener before filter pressing. Chemolysis solubilizes roughly 70-80% of the suspended solids. Oxidation is insignificant, as no oxygen or other oxidant is added. The solubilized matter, contained in the thickener supernatant and filter press filtrate, is then returned to the activated sludge system to be further converted to carbon dioxide and more microorganisms. The combination of increased conversion to carbon dioxide (in the activated sludge system) and improved dewaterability of residual solids has greatly reduced the mass of filter press sludge cake.
Wet air oxidation (WAO) is similar to heat treatment (that is, without acidification), but compressed air is injected to provide oxidation. WAO is generally performed at neutral or slightly alkaline (pH 8) conditions. Low pressure WAO is generally used to condition sludge to improve dewatering. For example, one WAO unit operated at 177 degrees Centigrade (350 Fahrenheit) and 2760 Kpa (400 psi) improved sludge solids so that a vacuum filter cake of over 40% solids was obtained without other conditioning. At those conditions, only about 5-10% of the organics (as measured by chemical oxygen demand) are removed and 17-31% of the suspended solids are solubilized. On the other hand, high-pressure WAO generally is operated at 240-300 degrees Centigrade (460-570 Fahrenheit) and 7000-14000 Kpa (1000-2000 psi) with a 40-60 minute retention. This oxidizes 60-80% of the organics and solubilizes 70-90% of the suspended solids. In addition, the greater level of oxidation provides enough energy from most sludges that operation is thermally self-sufficient, so steam is needed only for start-up. Because the level of solubilization exceeds the level of oxidation, a strong organic stream is still produced and returned to biotreatment. WAO is also used to oxidize inert or only slightly biodegradable aqueous streams prior to biological treatment.
A variation of WAO is the Barber-Colman "Wetox" process, which was intended to be operated with sulfuric acid (3 g per liter of sludge) at about 224-246 degrees Centigrade (435-475 Fahrenheit) and 4140 Kpa (600 psi). The response of different sludges can vary considerably. The addition of acid is said to improve the rate and extent of sludge oxidation and to prevent equipment scaling, which is often a major problem in WAO systems. The lower temperature and pressure, compared to high-pressure WAO, is said to reduce capital costs. Unlike most WAO systems, vapor and liquid are designed to be removed from a reactor separately to allow hot vapor to be used in a power generation scheme.
Chlorate oxidation of sludges is used in the pulp and paper industry. Conditions involve low pH (e.g. 2), higher temperature (e.g. 330-350 degrees Centigrade), and addition of chlorate and air (or oxygen). Unlike chemolysis, chlorate oxidation not only solubilizes suspended solids, but also oxidizes a large portion of the organics to carbon dioxide and other less oxidized species.
U.S. Pat. No. 3,984,311 discloses a process to reduce organic content in solution by using a oxidizing agent, such as air, in a co-catalyst system of nitrate with iodide and/or bromide ions. The conditions include relatively low pH (not more than 4, especially not more than 1) and temperature of above 100 degrees Centigrade, especially 150-300 degrees Centigrade. U.S. Pat. No. 4,212,735 discloses a similar process in which a catalyst system includes a transition metal.
There has long been a need for an effective and efficient sludge treatment process, particularly one for treating waste activated sludge and/or for pretreating aqueous streams that are difficult to biodegrade. There has long been a need for such a process which is efficiently operable at relatively lower temperatures and pressures, resulting in reduced capital costs. There has long been a need for such a process in which odor levels are significantly reduced. There has long been a need for such a process which results in reduced equipment corrosion. There has long been a need for such a process which results in an acceptably high level of solubilization of suspended solids and in high levels of oxidation.